1. Field of the Invention
The present invention relates to a control device for an engine that avoids or suppresses the occurrence of surging by retarding an open/close timing of an intake valve using a variable valve timing mechanism and advancing an ignition timing when surging is determined to have occurred.
2. Description of the Related Art
In a conventional engine installed with a supercharger such as an exhaust turbo-supercharger or a mechanical supercharger, a knocking limit decreases when a boost pressure is increased, thereby increasing the likelihood of knocking. To avoid a decrease in the knocking limit, control is performed to retard the ignition timing, but as retardation of the ignition timing progresses, a delay occurs in combustion ignition, leading to a decrease in the peak value of an internal cylinder pressure in comparison with ignition at the normal ignition timing, and as a result, the timing of the internal cylinder pressure peak is delayed. These phenomena occur during each combustion cycle such that variation in the peak value of the internal cylinder pressure and the timing of the internal cylinder pressure peak increases between each cylinder.
This variation in combustion between each cylinder leads to torque variation, and as a result of this torque variation, surging occurs. To avoid surging, means such as the following may be considered.    (1) Advancing the ignition timing by reducing the actual compression ratio (actual compression pressure).    (2) Reducing the set boost pressure.    (3) Advancing the ignition timing by enriching the air-fuel ratio.    (4) Increasing the combustion speed to raise the knocking limit.
Of these means, realizing (4) requires design modifications to the engine itself, and therefore (4) is not feasible. As a result of (1) and (3), the fuel economy deteriorates. With (2), the power performance, including the engine response, deteriorates.
In an engine with a supercharger comprising a variable valve timing (VVT) mechanism for varying the open/close timing of at least one of an intake valve and an exhaust value, a technique for avoiding knocking by controlling the intake timing has been proposed, as disclosed in Japanese Unexamined Patent Application Publication S61-187528, Japanese Unexamined Patent Application Publication S61-190147, Japanese Unexamined Patent Application Publication H6-330776 and Japanese Unexamined Patent Application Publication 2004-360552, for example.
Japanese Unexamined Patent Application Publication S61-187528 discloses a technique of avoiding knocking by setting an intake retardation amount in accordance with a map of the engine rotation speed and the boost pressure, and adjusting the retardation amount of the opening timing of the intake valve in each cylinder.
Japanese Unexamined Patent Application Publication S61-190147 discloses a technique of avoiding knocking by retarding the intake timing when the supercharger is operative and advancing the intake timing when the supercharger is halted. Japanese Unexamined Patent Application Publication H6-330776 discloses a technique of suppressing knocking by retarding the intake timing when the engine rotation speed is in a low speed region or a high speed region and advancing the intake timing when the engine rotation speed is in a medium speed region. Japanese Unexamined Patent Application Publication 2004-360552 discloses a technique of suppressing knocking by controlling the intake timing such that in a high load region, an effective compression volume increases as the load increases.
In all of the techniques disclosed in the patent publications described above, knocking is suppressed by retarding the open/close timing of the intake valve when knocking occurs. However, none of the publications consider measures to be taken when surging occurs.
FIG. 12 shows a relationship between an engine rotation speed Ne and a target boost pressure when a throttle valve is fully open. The reference symbol Pi denotes an intercept point, and in a higher engine rotation speed region than this region, increases in the boost pressure are restricted. Surging occurs easily in a transition period during which the boost pressure increases, and a region indicating by shading, which is surrounded by a surging limit line indicated by a broken line and a transition target boost pressure indicated by a solid line, serves as a surge region. Particularly large surging occurs in the vicinity of the intercept point Pi.
To improve the power performance and boost pressure response, the open/close timing of the intake valve is preferably advanced (the intake valve is closed at an earlier timing) even when the boost pressure is in the surge region, enabling an improvement in volumetric efficiency and an increase in torque. In this region, however, surging is likely to occur, and therefore, in actuality, the occurrence of surging is suppressed by reducing the boost pressure and in certain cases enriching the air-fuel ratio.
As a result, the power performance and the boost pressure response decrease. Furthermore, when the air-fuel ratio is enriched excessively, the original potential of the engine cannot be exhibited sufficiently, and the fuel economy deteriorates.